1. Field of the Invention
The present invention generally relates to photolithiographic processes in the manufacture of semiconductor devices and, more particularly, to using a latent image grating to determine quantitatively the exposure dosage required in semiconductor photolithography for optimum processing.
2. Description of the Prior Art
In photolithographic processes, the photoresist line-width variation typically needs to be controlled within 10% of its nominal value. Very often this cannot be achieved due to the interference effect which causes the energy absorbed by the photoresist to fluctuate. The interference effect is caused by the thickness variation of the photoresist itself, the thickness variation of underlying film(s), and the optical property change of the underlying film(s), e.g., due to hot processes. This problem is aggravated when the exposure sources use short wavelengths, i.e., i-line and deep ultra-violet (DUV) wavelengths. Small thickness changes can correspondingly result in large variations in absorbed energy.
Several methods have been used in the past to address the problem, among which are a "send ahead" wafer for each job. In this method, a series of dosages are used on each wafer, the wafer is developed and the line-widths corresponding to various dosages are measured to determine the optimum dosage. This method is accurate but time consuming and expensive.
A similar method was proposed by O. D. Crisalle, R. A. Soper, D. A. Mellichamp, and D. E. Seborg in "Adaptive control of photolithiography", SPIE, vol. 1464, pp. 508-526 (1991), with the same drawbacks. The problem can be eliminated with the addition of a top or bottom anti-reflective layer. This solution, however, adds an extra processing step and the material used makes the process more expensive.
J. A. Bruce, R. K. Leidy, M. S. Hibbs, and B. J. Lin in "Characterization and prediction of line-width variation due to thin film interference effects", Proceedings KTI Microelectronics Seminar, pp. 1-13 (1989), describe a process in which the reflectivity from the wafer surface can be measured and used to determine qualitatively if a higher or lower dosage needs to be used for that wafer or job. However, this method is effective when only one film thickness is varying. Otherwise, an infinite number of thickness combinations corresponding to different absorbed energies can result in the same reflectivity, in which case it cannot be used to determine the dosage.
Diffraction efficiencies of latent image (LIM) grating can be measured and used in conjunction with a rigorous Maxwell's equations' solver to determine the exposure dosage, as described by K. C. Hickman, S. M. Gaspar, K. P. Bishop, S. S. H. Naqvi, and J. R. McNeil in "Use of diffracted light from latent images to improve lithography control", SPIE, vol. 1464, pp. 245-257 (1991). However, one needs to know ahead of time the cause (e.g., which film thickness is varying) of the line-width variation and the optical indices of all relevant materials, so that the solver can perform the calculation. Neither of the two requirements can be easily achieved in production.
Nomarski differential interference contrast (NDIC) microscopy can be used to measure LIM to determine the exposure dosage. Commercially available NDIC microscopes can be modified to perform this task. In-situ, real time dosage adjustment can be done by incorporating such hardware into a stepper.